Ring communication system and method for connecting a plurality of end stations

ABSTRACT

A method of connecting a new end station to a ring communication system for connecting a number of end stations is constituted by the steps of: i) sensing an attempt by the new end station to attach to the ring; ii) determining whether there is a single end station only existing on the ring; and, iiia) if it is determined that there is more than one end station on the ring, allowing the new end station to attach to the ring and maintaining the ring communication configuration; or b) if it is determined that there is a single end station only on the ring, breaking the ring configuration and setting up two separate communication paths between the new end station and the existing end station.

FIELD OF THE INVENTION

The invention relates to a method of operating a ring communicationsystem, for example based on a token ring or FDDI communicationprotocol.

DESCRIPTION OF THE BACKGROUND ART

In a conventional ring based data communication system a token passingor similar scheme is used to control access to the ring by end stations.These end stations are typically then connected together via a hubcontaining control systems and the like.

Ring communication systems can support large numbers of end stations butfor small numbers of end stations, particularly two end stations, thecommunication protocol can slow down network throughput.

Where only two end stations are communicating in the network, it ispreferable to establish two way communication between them (known as"full duplex mode").

EP-A-0439008 discloses a method of operating a token ring network. In afirst "full duplex auto-configuration" mode the ring is initialized inconventional token ring mode, followed by a "2-station test" whichascertains whether only two stations are active in the network. If morethan two stations are active in the network, token ring operation ismaintained. If only two stations are active, the network switches tofull duplex mode. This is an inefficient way of operating the network,and does not allow a new end station to enter the network in full duplexmode. In a second "full duplex fixed" mode, the network is initializedin full duplex mode without any "2-station test", and without revertingto token ring mode in the event of a third station's presence orcessation of full duplex operation by the other station. This is afairly crude method, which does not allow conventional token ringoperation.

SUMMARY OF THE INVENTION

In accordance with one aspect of the present invention, a method ofconnecting a new end station to a ring communication system forconnecting a number of end stations comprises:

i) sensing an attempt by a new end station to attach to the ring;

ii) determining whether there is a single end station only existing onthe ring; and,

iiia) if it is determined that there is more than one end station on thering, allowing the new end station to attach to the ring and maintainingthe ring communication configuration; or

b) if it is determined that there is a single end station only on thering, breaking the ring configuration and setting up two separatecommunication paths between the new end station and the existing endstation.

In accordance with a second aspect of the present invention, a ringcommunication system comprises communication means for connecting anumber of end stations together in a ring configuration; and controlmeans for carrying out a method according to the first aspect of theinvention.

This invention specifically addresses the case where a new end stationwishes to join a ring having only a single end station and enables theaggregate bandwidth of the network in that case to be doubled by settingup two separate or independent communication links between the stations.The negotiation allowing full duplex operation is undertaken before theend station has entered the ring.

In some cases, the use of two separate or independent communicationlinks can be used to enable relatively long cables or poorer qualitycables to be used. Thus, crosstalk and other interference which canoccur between adjacent links in a cable can be minimized by arrangingfor only one of the communication paths to be active at any one time.

If the system is operating in the "separate path" or full duplex mode,then an additional station can only attach if the system reverts tonormal ring based operation. For example, if one of the stationsoperating in full duplex mode detects an initialization frame from athird station then it will cause communication to revert to a ringconfiguration.

In many such systems, a hub unit is provided typically with an endstation located within the hub unit. In that case, the end stationwithin the hub unit will constitute the existing end station.

Preferably, the method further comprises causing the new end station toemit data indicating whether it is capable of operating in the "separatepath" mode. This then enables the system to decide whether it shouldthen carry out steps ii) and iii). The present invention allows the hubunit to be a dual mode hub which can be used with end stations whichcould be normal token ring stations, full duplex only end stations, ordual mode stations (full duplex and normal token passing).

BRIEF DESCRIPTION OF THE DRAWINGS

An example of a ring communication system and method according to theinvention will now be described and contrasted with a conventionalsystem with reference to the accompanying drawings, in which:

FIG. 1 illustrates a conventional ring topology;

FIG. 2 illustrates a topology according to an example of the invention;

FIG. 3 illustrates an intermediate stage in the joining of a new endstation to an existing ring according to an example of the invention;and,

FIG. 4 illustrates a topology according to another embodiment of theinvention.

DETAILED DESCRIPTION OF A PREFERRED EMBODIMENT

The example to be described will be based on a token ring implemented oncopper cables in a conventional hub based configuration.

FIG. 1 illustrates a conventional token ring system, for exampleimplementing the IEEE 802.5 protocol. The system comprises a hub unit 1which controls communication around a ring 2 between end stations 3,4.In practice, single cables will link the stations 3,4 to the hub unit 1with the ring being defined within the hub unit 1. As can be seen inFIG. 1, a connection exists between both stations 3,4 but at any instantdata or control information will only be passing between station 3 andstation 4 or station 4 and station 3.

The invention proposes that in the specific case where there are onlytwo end stations 3,4 on the ring then the ring should be broken todefine separate communication paths 5,6 along each of which data can betransmitted. The two links 5,6 operate independently of each other thusdoubling the aggregate bandwidth of the network.

For convenience of operation, data frames passed between the two endstations could still be formatted as token ring frames so that anyend-user applications are unaware of the new mode of operation of theend stations.

In some cases, the end stations 3,4 could be manually configured tooperate in this "separate path" or "full duplex" mode but in other casesit is desirable to allow individual end stations to signal that they cansupport the full duplex method of operation in addition to the standardring based protocol. An end station then may operate automatically ineither mode.

FIG. 3 illustrates an existing ring communication system 7 of which thehub unit 1 forms a part. An end station 8 is to be attached to thesystem 7. Initially, the end station 8 is connected to the hub unit 1via a central control system 9 so as to define a subsidiary ring 10. Thesystem then operates as follows:

1) The end station 8 completes all its internal self tests. At thispoint, the two rings 7,10 are not joined at the hub 1.

2) The station 8 then sends test data to verify that the cable betweenthe end station 8 and the hub 1 is operational. This data is sent aroundthe ring 10 to the control system 9 and back along the ring to the endstation 8. This is achieved typically via relays within the controlsystem 9.

3) The end station 8 then sends a DC current along the ring 10 to thecontrol system 9 to command the control system 9 to join the rings 7,10and the end station 8 then enters the ring 7.

In a modified procedure according to an example of the presentinvention, the process is as follows:

1) The end station 8 completes all its internal self tests and at thisstage the two rings 7,10 are not joined.

2) The station 8 sends test data to verify that the cable between thestation 8 and the control system 9 is operational. The data is sentaround the ring 10 and returns to the end station 8. Within the testdata, the end station 8 indicates that it is capable of entering fullduplex mode. This can be achieved by including a predefined series ofcharacters within the test data. Alternatively, the end station couldassert a DC phantom drive signal in a non-standard manner to indicateits ability to act as a full duplex end station.

3) The control system 9 within the hub unit 1 may be unwilling to allowfull duplex mode operation. This may be because there are alreadyseveral stations on the ring 7 or it is unable to recognize thepre-defined data pattern. In that case, the end station 8 joins the ring7 in the conventional manner as described above (see step 3).

4) If, however, the control system 9 is able to allow the end station 8to join in the full duplex mode (because there is only one other endstation on the ring 7) then the control system 9 modifies the test datafrom the end station 8 and returns it around the ring 10 to the endstation 8. In other words, the test data from the end station 8 is notsimply looped at the control system 9 but read, modified and sent backto the end station 8. Alternatively, the control system 9 could send anadditional message to the end station 8.

5) The end station 8 then sends a DC current along the transmit side ofthe ring 10 to the control system 9 to join the rings 7,10 in the fullduplex mode similar to that shown in FIG. 2.

Step 5 is optional rather than necessary, as in most cases in fullduplex mode the hub 1 would have to act as a LAN bridge for a fullduplex station. Step 4 would then be sufficient to allow the hub to moveframes onto the "backbone" ring.

The above description is applicable to token ring communicationprotocols but with suitable modification could be used with other ringcommunication protocols such as FDDI.

A fully automatic system has been described above in connection withFIG. 3 but in other configurations the system could be manuallytransferred to full duplex mode.

The end station on the ring 7 is not shown in FIG. 3 and in some casesan end station 11 could be located within the hub unit 1, as illustratedin FIG. 4, for example.

We claim:
 1. A method of connecting a new end station to a ringcommunication system, having a ring communication configuration, forconnecting a number of end stations, the method comprising:i) sensing anattempt by the new end station to attach to the ring; ii) beforeallowing said new end station to attach to the ring, determining whetherthere is a single end station only existing on the ring; and, iiia) ifit is determined that there is more than one end station on the ring,allowing the new end station to attach to the ring and maintaining thering communication configuration; or b) if it is determined that thereis a single end station only on the ring, breaking the ringconfiguration and setting up two separate communication paths betweenthe new end station and the existing end station.
 2. A method accordingto claim 1, wherein the new end station generates pre-defined data toindicate its ability to operate in a "separate path" mode.
 3. A methodaccording to claim 1, wherein the single end station is located within ahub unit.
 4. A ring communication system having a ring communicationconfiguration and comprising communication means for connecting a numberof end stations together in a ring; and control means for carrying out amethod according to the following steps:i) sensing an attempt by a newend station to attach to the ring; ii) before allowing said new endstation to attach to the ring, determining whether there is a single endstation only existing on the ring; and iiia) if it is determined thatthere is more than one end station on the ring, allowing the new endstation to attach to the ring and maintaining the ring communicationconfiguration; or b) if it is determined that there is a single endstation only on the ring, breaking the ring communication configurationand setting up two separate communication paths between the new endstation and the existing end station.
 5. A system according to claim 4,further comprising a hub unit, the control means being situated withinthe hub unit.
 6. A system according to claim 5, wherein a single endstation is located within the hub unit.
 7. A system according to claim4, further comprising a hub unit, wherein the hub unit is adapted foruse with normal token ring stations, full duplex only end stations ordual mode stations.